But despite all the buzz, Coca-Cola and PepsiCo's plant-based bottles are still very much plastic.The companies have merely replaced the fossil fuels (petroleum and natural gas) traditionally used to make their plastic bottles with ethanol from renewable sources (plant waste in Pepsi's case and Brazilian sugar cane in Coke's). Though these initial inputs come from renewable, lower-carbon sources, the resulting plastics are chemically identical to the polyethylene terepthalate, or PET,and high-density polyethylene, or HDPE, that regular plastic bottles are made of—a fact the companies acknowledge. And once the inputs become plastic, they carry all the same environmental impacts as plastic made from fossil fuels: They don't biodegrade, they pollute the world's oceans and soils, and still leach potentially harmful chemicals into our food.

"They're just using plants to make the same polymers you find in other plastics. It has zero effect on plastic pollution," says Marcus Eriksen, a marine expert who co-founded the nonprofit 5 Gyres a few years ago to study ocean plasticization in areas like the Great Pacific Garbage Patch.

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Eriksen and his crew just finished exploring the world's five major oceanic gyres (slow-moving currents that create massive whirlpools where plastic can accumulate). They've found "plastic soup"—water thick with tiny bits of plastic—in all five. Eriksen's team and other researchers have also found larger chunks of plastic on the various islands scattered throughout the gyres, and in the bellies of dead birds, fish, and animals who fill themselves up with plastic bits that they mistake for fish and eventually die because they can't digest the stuff.

Likewise, plastic—plant-based or otherwise—harms human health. The dangers of chemical additives commonly used in plastic, such as BHT, * as well as chemical compounds released by plastics, such as acetaldehyde have been widely publicized for their apparent link to various types of cancer.

"Some bioplastics formulations use the same types of additives as petroleum or natural gas-based plastics," acknowledges Melissa Hockstad, a vice president at SPI, a trade association for the industry. In other words, plant plastics are not necessarily free of harmful chemicals. There's no way to know whether a particular plant-based plastic bottle includes these chemicals, since all plastic "recipes" are protected as trade secrets. But since traditional PET and HDPE manufacturers tend to use them to produce the right level of pliability and clarity, there's a very good chance that plant-based versions of PET and HDPE contain them, too. Hockstad says "some companies have been working on the development of bio-based" alternatives. But the key phrase is "working on the development of," as in, those additives don't exist yet and may never.

That said, there's a kernel of real progress amid the plant-plastic hype. The new bottles reduce the use of fossil fuels and improve recyclability. But there's a big difference between "recyclable" and "recycled." While all bioplastics are technically "recyclable," current recycling systems are not set up to recycle those that don't mimic existing plastics. The most common bioplastics include polylactic acid, which is made from corn starch, tapioca, or sugar cane. When these bioplastics arrive at a recycling center, they are separated out as waste.

In this sense, Coke and Pepsi opting to create plant-based HDPE and PET instead of other bioplastics is applaudable. Unfortunately, people still recycle only a small fraction of the plastic bottles they use, regardless of how those bottles are made. (Manufacturers typically put the recycling rate for PET at 27 percent, while recycling advocates suggest it's more like 21 percent.) Most plant-based bottles, sadly, will end up in landfills or along the side of the road.

As such, it's crucial not to misread plant-based as biodegradable. "As a recycler, I'm much happier with the bioresins that we're able to recycle, but I don't want it to turn into something where people think because they're buying a plant bottle, they can be wasteful," says Gerry Fishbeck, vice president of the United Resource Recovery Corp., a large recycling company that has a partnership with Coca-Cola.

Pepsi and Coke could do a greater environmental good by focusing on recycling instead of on making plant-based bottles. They could, for instance, finally throw their support behind bottle bills, state legislation that creates a deposit system for beverage containers. In the 10 states where they're enacted, bottle recycling rates range from 60 percent to 80 percent, dramatically higher than the national average. But so far Coca-Cola and Pepsi have balked at bottle bills, claiming they create an unreasonable business cost. (Under the bills, manufacturers are required to set up the deposit systems as well as arrange for the collection and processing of their empty containers.)

They should also commit to using recycled material in their packaging. In 2001, Coca-Cola pledged to use 10 percent recycled plastic in their PET bottles by 2005. It achieved that goal, promptly dropped its commitment to using recycled content, and now evades precise accounting with statements like, "We are working to advance technologies that allow us to use greater amounts of recycled materials in our packaging." The bottles currently produced by major bottling companies use about 4 percent recycled content, according to Susan Collins, executive director of the Container Recycling Institute.

There's no question that setting up collection facilities and integrating recycled material into the production stream is expensive. But several companies are already doing it, Collins says. Naked Juice, Naya Water, Eldorado Water, and Rainbow Light Nutritional Systems, for example, are all using 100 percent recycled plastic in their bottles. Meanwhile, Coke and Pepsi are spending millions on a plant-based version of the same old packaging and touting it as an environmental leap forward. Don't be fooled: It's a sidestep at best.

Correction, June 15, 2011: This article incorrectly suggested that bisphenol A and pthalates are commonly used as additives to PET and HDPE plastics. (Return to the corrected sentence.)

Amy Westervelt is a freelance environmental reporter based in Oakland, Calif.